FIG. 1 (Prior Art) is a diagram of a conventional TRIAC dimmable LED driver. As shown in FIG. 1, an alternating-current (AC) voltage source 101 is in series with a TRIAC dimmer 102, and is connected to the input terminal of a diode bridge 103. The diode bridge 103 rectifies the input AC voltage and generates a sinusoidal voltage on capacitor C1. Resistor R4 and capacitor C2 generates a DC voltage to start up the control IC 104.
The transformer T1 has three windings, including a primary winding connected with capacitor C1 and the drain of the power M1, a secondary winding connected with a rectifying diode D1 and a secondary output capacitor C4, and an auxiliary winding connected with a supplying diode D2 and the circuit ground.
After the circuit starts up, the auxiliary winding powers up the control IC 104. At the same time, the auxiliary winding provides the rectifier diode D1 zero-crossing current information and the output over voltage information.
The prior art usually contains a resistor divider R2 and R3, which is used to detect the TRIAC phase cutting angle.
The prior art also contains a bleeding resistor R1 and a bleeding switch M2 . When the input current of the flyback converter is small, the control IC 104 turns on the bleeding switch M2, providing enough holding current for the TRIAC dimmer.
The prior art has the following disadvantages:
First, the bleeding resistor R1 and bleeding switch M2 dissipate too much power, losing the LED driver efficiency and inducing too much heat . The LED driver suffers from high temperature and low reliability.
Second, the control circuit is complex, and the system cost is relatively high.